The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.
Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) represents the highest spectral performance mass analyzer available. The high mass resolving power, high mass accuracy and high dynamic range enable high performance mass spectrometry from small molecules to intact proteins. Many tandem MS capabilities are amenable with FT-ICR MS, which include collision-induced dissociation (CID, both external and in-cell), infrared multi-photon dissociation (IRMPD), electron capture dissociation (ECD) and electron transfer dissociation (ETD). FT-ICR MS is the analyzer of choice for precision proteomics, top-down proteomics, mass spectrometry imaging and petroleomics.
In the presence of a strong magnetic field, an ion gains a rotational frequency that is inversely proportional to its mass. The relation is given by the cyclotron equation ωc=qB/m, wherein ωc is the so-called unperturbed ion cyclotron frequency, q is the ionic charge, m is the ionic mass and B is the magnetic field strength. The ICR frequencies for common biological molecules fall in a very suitable range for commercial electronics (kHz to MHz range). The frequency of the ion cyclotron rotation can be measured very accurately, thus the mass measurement accuracy for FT-ICR MS is very high, easily in the parts-per-million or parts-per-billion range. This high mass accuracy allows for elemental composition assignment and thus improved identification capabilities over lower performance mass spectrometers. Similarly, since the detection period for FT-ICR is long and the frequency measured is high, the mass resolving power is high. This enables spectral separation of very closely spaced mass peaks.
One of the key challenges in mass spectrometry is the ability to accurately identify the molecular species analysed. The lack of mass resolving power can result in obscured spectral details when two closely neighbouring peaks are not resolved in the mass spectral domain. The high mass resolving power of FT-ICR MS easily resolves closely spaced mass peaks within less than 5 mDa (0.005 Da) mass units that would be difficult to resolve with lower performance mass spectrometers.
The cyclotron equation defines the “unperturbed” ion cyclotron frequency. However, experiments necessitate the axially confinement of ions in the FT-ICR analyzer cell. The application of a small electrostatic “trapping” potential affects the ion cyclotron measured frequency. Further, the presence of other m/z ions in the FT-ICR analyzer cell can lead to ion-ion interactions (or space charge effects), which further distort the electric field in the FT-ICR analyzer cell and thus change the ion cyclotron frequency.
To these ends, the simulation of ion motion in a FT-ICR analyzer cell has become a popular method for the study of these experimental necessities. Phenomena studied by simulation include, amongst others, ion cloud motion, peak coalescence and ion-ion interactions (space charge). Such high-performance computer simulations are powerful to examine the theoretical behaviour of ions inside the FT-ICR analyzer cell. The simulations allow important parameters (magnetic field, number of ions in the FT-ICR analyzer cell, electrostatic potentials, electrostatic trapping field geometry, starting position of the ions prior to excitation, excitation radius, excitation waveform type, etc.) to be varied. These simulations are however intrinsically limited by the computing time (which correlates with the accuracy of the modelling parameters). Understanding of the effects of these parameters on FT-ICR signal quality has led to new FT-ICR cell design geometries as well as important insight into FT-ICR experimental parameters for improved data quality and insight in the fundamentals of the ion motion. However, simulation results remain theoretical and comparison with experimental data is highly desirable.
From the above it follows that the known detection techniques for detecting ions in an ion trap suffer from a number of disadvantages that do not allow these detection techniques to be used as a useful tool in ion-trap applications such as FT-ICR spectrometry. Hence, there is a need for improved systems and methods for detecting ions in an ion trap. In particular, there is a need for improved systems and methods for detecting the spatial distribution and/or important parameters of ions in an ion trap inside a strong magnetic field and (ultra) high vacuum.